The invention relates to an apparatus for detecting signal in a radio communication system, and more specifically to a radar signal detecting apparatus for detecting an adjacent frequency radar signal.
According to rapid progress of cellular phone on personal use, needs for radio communication have been increasing. Also, according to rapid progress of internet through radio, not only conventional speech communication but also data communication has been increasing and lack of radio frequency has been problem. In such a background, as one solution to the problem, it has been studied to use a new system together with a conventional system without interference in the same radio bandwidth. This is an essential subject from perspective for utilization of frequency.
Since transmitting signal from own system may interfere with other systems, it is a prerequisite condition that a new system does not interfere with a conventional system.
Generally, a radio apparatus in the new system is designed in consideration of spatial distance between the radio apparatus and an adjacent radar receiver and a transmitting signal level of the radar is measured for foundation of a new office to avoid interference of a transmitting signal of the radio apparatus in the new system to an adjacent radar receiver. But the measurement at the new office needs a lot of time and money, and when the radar performance and radio environment are changed, another measurement and design are needed. Moreover, establishment of many radio apparatuses will be the cause of complicated control.
Therefore, it is needed that a radar signal is measured automatically while the radio apparatuses are in use.
Concerning interference of a transmitting signal of the radio apparatus to the radar receiver, it is possible to estimate interference from the radio apparatus to the radar by measuring a communication characteristic from the radar to the radio apparatus. This method utilizes radio reversibility which can measure the communication characteristic if the radio apparatus receives a radar transmitting signal level for estimating the communication characteristic. It is possible to know whether the transmitting signal of the radio apparatus influences the radar and permissible transmitting power of the radio apparatus by estimating communication characteristic. And it is possible to stop transmitting from the radio apparatus and avoid interference at the worst. But there are problems hereinafter when the radio apparatus detects the radar signal level.
The radar has alternate time slot for transmitting and receiving signal. The transmitting signal is pulse shaped and has a short cycle. A reflected signal from an object is detected in the receiving time slot. Generally, the radar is rotated and detects the reflected signal. Therefore, the radio apparatus has to detect the pulse shaped signal and can not detect the radar signal during a receiving period. And the radio apparatus can only detect the radar signal brokenly because the radar rotates during the receiving period. For example, when a radar signal detecting level reaches to some level for detecting interference, the radio apparatus tries to avoid interference. But it is possible that the radio apparatus can not detect a radar level exactly because of a relation between the transmission interval of the radar signal and the rotational velocity of the radar. Especially, it is difficult to detect the radar level when a radar antenna has high gain and sharp directional characteristic.
If the radio apparatus which detects the radar signal level has a transmitting function, another problem will occurr. When the radar detecting level comes to low, it will be impossible to detect the radar signal level during the transmitting period of the radio apparatus because of interference of the apparatus"" own transmitting signal.
When the radio apparatus uses TDD (Time Division Duplex) method, which repeats transmitting and receiving one after the other, for telecommunication as the radar, the radio apparatus needs to detect the radar signal separately from the repetition of the transmitting and receiving signals. But since the radio frequency of the radio apparatus and the radar are close, it is difficult for the radio apparatus to detect the radar signal during its own transmitting period. In this case, the radio apparatus can detect a radar signal during its own receiving period only when a radar transmits signal.
FIGS. 6 and 7 show transmitting and receiving timing of the radar and transmitting and receiving timing of the radio apparatus using TDD method. As shown in FIG. 6, when transmitting and receiving timing of the radar and the radio apparatus are synchronized completely, the radio apparatus can detect all transmitting signal from the radar. This is an unusual case. If the radio apparatus does not try to synchronize with the radar, it is impossible that the radio apparatus can detect all transmitting signal from the radar. Therefore, as shown in FIG. 7, it will be occur detectable time period and undetectable time period.
FIG. 8 shows variation of a receiving level at the radio apparatus, when the radar rotates. When the radar turns straight to the radio apparatus, the receiving level comes to maximum, and the receiving level changes suddenly near this angle. This is a case of sharp radar antenna characteristic. As shown in FIG. 8, it is difficult to detect the radar signal when the threshold or detecting range used by the radio apparatus are in the changeable period of the receiving level. Also as shown in FIG. 9, if the transmitting timing of the radar signal accords with the detecting range, it is difficult to detect the radar signal for interference of its own transmitting signal when the transmitting timing of the radar is same as the transmitting timing of the radio apparatus.
It is possible to spread a level detecting range to detect the radar signal, but system performance declines since a timing for halting signal or power control is too fast. For example, a maximum increase of antenna gain is about 10 dB per one cycle with parabola 4 meters in diameter directivity and 4 ms pulse cycle of the radar signal. This shows that beam width of the radar antenna is narrow and the detecting level in the radio apparatus changes with a range of 10 dB during pulse cycle of the radar signal in consideration of rotational velocity. The signal level will change with a range of 20 dB until next detecting timing by losing one detecting timing. Therefore, the radio apparatus needs to detect the radar signal every time and detects it with own transmitting timing or detect it by halting transmitting signal in a time period which is possible to receive the radar signal.
When the radio apparatus detects the radar signal by halting transmitting signal in the time period which is possible to receive a radar signal, the radio apparatus needs to halt signal with extra 20 dB margin once the radio apparatus has structure which can not detect the radar signal. Therefore, threshold for detecting the radar signal has to be lowered in consideration of the extra margin. It is difficult for the radio apparatus to detect all transmitting signals from the radar during own communication, and interference of the radio apparatus to the radar is a problem coming from radar rotation.
In order to address this need, the present invention provides a radar signal detecting apparatus for detecting an adjacent frequency radar signal.
According to one aspect of the present invention, for achieving the object, there is provided, as a specific configuration, a radar signal detecting apparatus including a coupler diverging a part of a signal from a received signal, a local oscillator outputting a local signal, a mixer converting the diverged signal multiplied the local signal into low frequency signal and a detector detecting a radar signal from the low frequency signal by using a threshold above a side lobe.
Preferably, the detector detects the radar signal by using a detecting level range between the threshold and below the side lobe, and halts or attenuates a transmitting signal when a received signal level is over the threshold.
Preferably, the coupler is a directional coupler comprised of micro strip line, and the directional coupler has tight coupling for signal from an antenna and loose coupling for signal from a transmitter.